In recent years it has become commonplace to manufacture synthetic silica glass by vapour deposition processes, wherein the vapour of a silicon compound is fed to a burner, optionally mixed with the vapour of one or more dopant precursors, and converted to oxide nanoparticles in the flame. These particles are then deposited, either as a porous body (known as a soot body) which can be sintered subsequently to pore-free glass, or at higher temperatures to form a glass directly, the so-called direct quartz process.
The requirement for volatile precursors has led to the widespread use of silicon tetrachloride as the major silica precursor, and a variety of metal and non-metal chlorides as precursors for dopants, where doping of the product is required.
However, there are several advantages in using volatile chlorine-free silica precursors in place of silicon tetrachloride, in particular the absence of acidic by-products in the effluent gases. The potential use of alkoxides was recognised in some of the earliest work on synthetic silica (e.g. U.S. Pat. No. 2,272,342), but more recently the siloxanes have been used as combustible chlorine-free precursors with very high silicon content, notably hexamethyldisiloxane (HMDS), octamethylcyclotetrasiloxane (OMCTS, also known as D4), and decamethylcyclotetrasiloxane (DMCPS, or D5), see e.g. EP 0,463,045 and U.S. Pat. No. 5,043,002 (U.S. Pat. No. RE 39,535).
While vapour feed of the precursors is now regarded as conventional, there are a number of examples where the synthesis flame has been generated, not by feeding the precursors as vapour, but rather by feeding one or more of the precursors as an atomised spray of liquid droplets, which vaporise in the flame, and are then converted by oxidation or hydrolysis to silica nanoparticles.
Thus, the manufacture of pure and doped silica by deposition of a porous deposit from a spray of precursors into a flame was proposed in JP 54-142317, JP 55-95638, and JP 56-14438. A variety of oxy-hydrogen burner configurations was proposed in JP 55-95638. In most of these a liquid, which could be a mixture of a silica precursor and dopant precursors, was atomised by gas-blast (pneumatic or Venturi) atomiser into an oxy-hydrogen flame, and the products were collected as a porous soot body to be sintered to glass. Precursors of silica included tetraethoxysilane (TEOS), and tetramethoxysilane (TMOS), and dopant precursors included PO(OC2H5)3 and B(OC2H5)3. It was further suggested that metallic dopants could be introduced as separate sprays of aqueous or alcoholic solutions, and thus used to control the refractive index of the product glasses to make optical fibre preforms. Other applications envisaged included glass ceramics, and laser-active glass compositions.
The ability to feed a dopant species as an aqueous solution, rather than as a costly or unstable organometallic species, would permit significant cost reduction, but the quality of glass which would result from feeding two separate sprays into a flame must be open to question.
The atomisation approach was further developed in JP 56-14438, in which increased deposition rates were said to be possible if the pneumatic atomisation of the liquid precursor was replaced by the use of an ultrasonic atomising horn, or alternatively if pneumatic atomisation and ultrasonic atomisation were used in combination. A soot deposition rate of 400 g/h was reported, but the effectiveness of converting a spray comprising separate droplets of silica precursor and dopant is again questionable, and there are no indications of the quality of glass produced by this method.
Spray combustion for generating pure and doped silica glasses has been pursued by other workers. JP 60-96591 describes feeding a pneumatically generated spray of tetraethoxysilane (TEOS), or tetramethoxysilane (TMOS) into the centre of a co-annular oxy-hydrogen flame to provide a porous deposit of pure silica which could be sintered to glass. More recently, U.S. Pat. No. 5,110,335 proposed ultrasonic atomisation using SiCl4 or TEOS as precursor liquids, and combustion in an oxy-methane flame. Spray combustion of chlorine-free siloxane precursors has also been described in a number of patents, for the manufacture of both pure and doped synthetic silica glasses (e.g. EP 0,868,401, U.S. Pat. No. 6,260,385, U.S. Pat. No. 6,588,230, U.S. Pat. No. 6,374,642, U.S. Pat. No. 6,739,156 and U.S. Pat. No. 6,837,076).
It is notable that despite the number of patents in the literature which describe deposition of synthetic silica from a flame fed with a spray of liquid precursors, the majority of synthetic vitreous silica manufactured to date is still being made by combustion of precursors fed in the form of vapour. This is despite the advantages which might appear to result from a move to spray feed of a liquid precursor. For spray combustion, all that might appear to be required is the provision of a liquid storage and handling system, a metering pump, and the requisite gas supplies with appropriate flow controllers, together with a suitable burner, or array of such burners. For vapour feed, there is the additional need to provide suitable heated vaporisers for silica and dopant precursors, and appropriate heated lines for the supply of vapours to the burner, all to be equipped with appropriate temperature monitoring and control facilities. However, the manufacture of high quality glass by spray combustion is not without problems.
In U.S. Pat. No. 6,260,385 (column 2 line 61-column 3 line 8) it is noted that simple atomisation burners based on pneumatic (air-blast) atomisation may give unsatisfactory soot bodies with wart-like growth, unless the atomising gas is supplied at high velocity. This facilitates atomisation to smaller droplets and reduces the risk of incomplete vaporisation and combustion before the droplets strike the substrate, but the increased flame turbulence has the disadvantage of reducing the soot deposition efficiency, and can give rise to other problems. It was noted in this patent that these problems might be overcome by using ultrasonic atomisation; however this option was dismissed in favour of a so-called effervescent atomisation burner, in which a gas was dissolved under pressure in the precursor liquid, and as the liquid issued from the nozzle it was said to break up into smaller droplets as the dissolved gas formed bubbles in the emerging droplets. However, it appears that it was still necessary to surround the spray with some form of tubular containment (referred to as a “rail”), in order to achieve a satisfactory confinement of the flame for efficient deposition of silica soot. No examples were given of the application of this concept.
Further potential problems arising during spray combustion of siloxanes were discussed in U.S. Pat. No. 6,374,642. Defects in the soot body result if the precursor droplets are too large. They may strike the substrate before they are fully vaporised and then oxidation takes place on the substrate surface. Alternatively, partial vaporisation and oxidation may take place in flight, but then nucleation of silica may occur around the partially vaporised droplets, leading to oversized soot particles. For these reasons, an ultrasonic atomiser was incorporated in the burner, as a means to achieve fine droplet size, without the need for excessive flow of atomising gas. Additionally, the reacting plume from the synthesis burner was surrounded by flames from two or more auxiliary burners, which were intended to ensure complete vaporisation and combustion of the precursor, before reaching the substrate. These additions add greatly to the complexity of the process. Some indications were given regarding possible gas flows and operating distances between burners and substrate, but the quality of the resulting glass is not clear.
Where it is required to produce a doped synthetic silica glass in which the dopant oxide is refractory, i.e. has high melting and/or boiling point, and where it is required to make a glass uniformly doped at an atomic level, and free from bubbles and inclusions, the technique of spraying two liquids together into a flame raises further problems, unless the precursors can be fully vaporised and homogeneously mixed prior to, or in the course of, combustion. Otherwise one or both of the precursors can become converted to a solid or liquid particle whose size will depend on the droplet size in the spray but generally, in the absence of total vaporisation of all the relevant species and homogeneous nucleation and growth of doped glass particles in the flame, this technique cannot be expected to yield a doped glass of the required uniformity on an atomic scale. These points were noted in a recent patent (U.S. Pat. No. 7,624,596), but the practicality of the solution there proposed (supersonic expansion through a Laval nozzle) is unclear, and no examples were given.
One possible reason for the apparent failure of the atomisation approach is the relative difficulty of achieving a uniform spray, of uniform droplet size, in a uniform gas flow, and thus to achieve vaporisation of all droplets, and then controlled combustion in a well-defined region of the flame, as required to generate a high concentration of fully oxidised nanoparticles for collection on the substrate, uncontaminated by any unburnt particles of precursor, or oversized particles of oxide or of other products of pyrolysis of the precursor (e.g. silicon carbide or oxy-carbide) which might lead to defects in the product glass. In the past, these requirements have been achieved more readily in the flame of a synthesis burner fed with all precursor species in the form of vapour.
For deposition of doped silica using two or more precursors, there is the additional problem of ensuring intimate mixing of the constituent species, in constant and repeatable proportions. Where the precursors are available as miscible and chemically compatible liquids, they may be fed as a solution to a suitable atomising nozzle, and the resulting spray led to the soot deposition flame. Thus the manufacture of silica doped with tantalum oxide has been proposed via atomisation of a solution of tantalum butoxide in hexamethyldisiloxane (HMDS), to produce a tantalum-doped silica with increased refractive index (U.S. Pat. No. 6,546,757). The manufacture of silica soots doped with metal oxides, including rare-earth oxides, via spray combustion of organometallic compounds dissolved in siloxanes has also been described in U.S. Pat. No. 6,739,156. Proposed dopant compounds include alkoxides and β-diketonates. Mutual solubility may be enhanced by addition of a solvent (e.g. ethylene glycol monomethyl ether), but this patent notes that problems can occur because of blockage of feed-lines caused by the ready hydrolysis of the proposed organometallic dopant precursors. Possible solutions to these problems were offered but the use of costly organometallic dopant precursors means that this technique is unsatisfactory as an economically viable method of producing the required doped glasses on a large scale.
Furthermore, there are numerous applications for doped vitreous silica products where no suitable organometallic precursors exist, or where the high cost of using such compounds would be unacceptable. These include semiconductor components resistant to plasma etching, optical filters, specialised lamp envelopes, laser-active glasses, etc. While some of these applications have been served in the past by fusing natural quartz crystal powders intimately mixed with appropriate finely divided oxide powders, for certain applications today the methods previously used have proved to be inadequate. Where it is required to have the highest possible purity, combined with complete freedom from internal defects, from microbubbles, from any un-dissolved particles etc., it is desirable that the doped silica glass be manufactured from high purity synthetic feedstocks, and preferable that the glass be highly homogeneous, i.e. uniformly doped at an atomic level. In the past this has proved to be impossible to achieve on a large scale for many potential metallic dopants at an acceptable cost.
Particular difficulties arise when doping by certain more refractory oxides is required, and significant problems have been found with regard to homogeneous doping of silica with the oxides of certain transition metals, and especially with the oxides of the rare-earth metals. Furthermore, due to the difficulty in getting an ultimate solution of the rare-earth oxides in vitreous silica, because of their relative insolubility or “clustering” of the metal ions in the glass, it has become conventional to co-dope with aluminium oxide to aid the solubility and dispersion of the rare-earth metal ions.
As noted earlier, potentially volatile precursors of the rare-earth metals are either unavailable or very expensive, and are also difficult to handle. Thus it would be more cost-effective and more convenient if it were possible to use a relatively inexpensive salt of the relevant metal, or mixture of metals.
In US 2008/0039310 it was suggested that one precursor (silicon tetrachloride) could be fed as vapour, and a second precursor fed to the flame as a spray. In this case, an atomised spray of a solution of lanthanum and aluminium chlorides in aqueous ethanol was reported to have been fed into the flame of an oxy-hydrogen burner fed with SiCl4 vapour, and deposited as a porous glass on a substrate. This was suggested as a potential alternative to solution-doping of the soot body for the manufacture of such a doped glass. However, it would be difficult thus to ensure that the lanthanum oxide species is generated in the form of ultrafine particles. Unless complete evaporation of all precursor species occurs before conversion to oxide occurs, oxide formation begins in the condensed phase (in this case the dehydrated droplet of solution) and the dopant oxide will not be generated in the form of nanoparticles, as required for atomic level homogeneity in the glass. Furthermore, since the silica particles and dopant oxide particles are generated in two different regions of the flame, this approach is likely to give a soot-body in which the concentration of dopants fluctuates through the layers of soot, and will not give the ultimate homogeneity and/or transparency required in the most critical applications. It may be significant that while optical transmission data were provided in the above patent application for doped glasses prepared by solution doping of a porous body of silica soot, no such figure was given for material prepared by spraying a solution of dopant species into a separate silica synthesis flame.
The potential benefits of providing one or more dopants as an aqueous solution were again noted in U.S. Pat. No. 6,705,127, which proposed the provision of a non-aqueous silica precursor (e.g. a siloxane), optionally containing one or more soluble dopants (e.g. an organometallic compound), together with additional solvent if required, as an aerosol spray via one burner, together with a second spray, which consists of an aqueous solution of further dopants via a second burner. In this arrangement, the silica soot, and such dopants as exist in the second spray, were laid down separately in successive layers, which evidently cannot lead to the desired intimate mixture of components on an atomic scale. In another embodiment, it was envisaged that the non-aqueous and aqueous liquids were fed to a single burner, and thence sprayed into a single flame, but there was no indication as to the burner design in which this could be achieved, or if the technique was effective in providing a homogeneous deposit of acceptable quality.
US 2006/0001952 noted the requirement for the ultimate homogeneity of the glass, even when the dopant may be refractory and prone to form un-dissolved dopant particles or localised concentrations of the dopant oxide (i.e. “clustering”). For this reason it is desirable that, in the course of the combustion synthesis process, both silica and dopant precursor species become fully vaporised, and then condense together as nanoparticles of the mixed oxide particles. This should lead to intimate mixing at an atomic level. It was suggested that this could be achieved using a coaxial oxy-hydrogen burner, in which the central nozzle was fed with a mixture of aluminium, sodium and erbium chlorides in aqueous methanol, and this spray was atomised by a high velocity coaxial flow of hydrogen, itself surrounded by a flow of silicon tetrachloride vapour, and finally by a flow of oxygen. The gas velocities were said to be in the range 0.3 to 1.5 times the velocity of sound, and the high turbulence of the flame was said to give intimate mixing of the components. However, as reported in other patents, such high velocities and turbulence are not generally conducive to efficient collection of soot in the form of a uniform porous body free from defects and well suited to sintering to a defect-free glass, so while the approach may be suitable for deposition of small quantities of doped product, and capable of conversion to a high value optical fibre preform, from which usable material may be selected, it does not appear appropriate for the efficient manufacture of larger quantities of homogeneous defect-free glass.
While the use of salts of dopant species fed as atomised aqueous solutions to the flame has been proposed in the past, the approach is unlikely to lead to total vaporisation of all potential precursor species, especially where these are relatively involatile. Droplets of such solutions might be expected to evaporate to dryness, and then to decompose to solid or liquid particles, but because of the very high boiling point of the dopant oxides of particular interest, some at least of the resulting oxide particles will remain of significant size, and will constitute unacceptable defects in the product glass.
What is required is a method of synthesising the particles of dopant oxide, or oxides, in the flame, such that all the precursors are delivered to the reaction zone as a substantially homogeneous mixture, while ensuring that all the particles of each dopant oxide are either co-condensed from vapour species in the presence of condensing silica particles, or else generated as dispersed nanoparticles of dopant oxide of a size comparable to those which would be achieved by condensation from the vapour, i.e. of sub-micron dimensions.